Capacitance is the charge stored per unit potential difference between the plates of a capacitor.

The farad (F) is the SI unit of capacitance. Since 1 F is a very large unit, capacitance is usually quoted in microfarads (μF), nanofarads (nF) or picofarads (pF).

For a fixed potential difference, how does increasing a capacitor's capacitance affect the charge it stores?

The charge stored increases . The greater the capacitance, the greater the charge stored for a given potential difference.

A capacitor is commonly made of two conductive metal plates separated by a .......... , which prevents charge flowing between them.

A capacitor is commonly made of two conductive metal plates separated by a dielectric , which prevents charge flowing between them.

True or False? The charge Q in the capacitance equation is the total charge that has flowed around the circuit.

False. Q is the charge stored on the plates of the capacitor, not the total charge that has flowed through the circuit.

Define the electrical energy stored by a charged capacitor.

The work done by the power supply in moving charge onto the plates of the capacitor, which becomes stored as electrical potential energy.

On a graph of charge against potential difference for a capacitor, the gradient represents .......... .

On a graph of charge against potential difference for a capacitor, the gradient represents 1 / capacitance (1/C).

True or False? Adding the first few electrons onto the negative plate of a capacitor requires more work than adding the last few.

False. Electrostatic repulsion increases as the plate becomes more negatively charged, so more work is needed to add charge later in the charging process, not the first few electrons.

Define the time constant , τ , of a discharging capacitor.

The time constant is the time taken for the charge, current or potential difference of a discharging capacitor to decrease to 37% of its original value.

For a charging capacitor, what does the time constant represent?

The time taken for the charge or potential difference of a charging capacitor to rise to 63% of its maximum value.

During charging, how does the current in the circuit change with time?

It starts large and decreases exponentially to zero.

The rate at which a capacitor discharges depends on the .......... of the circuit.

The rate at which a capacitor discharges depends on the resistance of the circuit.

True or False? During charging, the p.d. across a capacitor increases exponentially in the same way the charging current decreases exponentially.

False. The p.d. and charge rise from zero towards a maximum value; only the current follows an exponential decay curve during charging.

How does increasing the circuit resistance affect the time taken for a capacitor to discharge?

A higher resistance means the current decreases more slowly, so the capacitor takes longer to discharge.

Define the resolution of a measuring instrument.

The smallest change in a quantity that a measuring instrument can detect. In this practical, the voltmeter has a resolution of 0.1 V and the stopwatch a resolution of 0.01 s.

What is the aim of Core Practical 11?

To calculate the capacitance of a capacitor experimentally, by discharging it through a resistor and measuring how the p.d. varies with time.

What are the independent and dependent variables in this experiment?

Independent: time, t . Dependent: potential difference, V .

How many voltage readings should typically be taken, and at what time interval?

8–10 readings , taken every 10 s until the p.d. reaches 0 V.

What two quantities are plotted against each other to obtain a straight-line graph for finding capacitance?

ln( V ) is plotted on the y -axis against time, *t , on the x*-axis.

The capacitance of the capacitor can be calculated from the .......... of the ln(V) against t graph, since gradient = −1/RC.

The capacitance of the capacitor can be calculated from the gradient of the ln(V) against t graph, since gradient = −1/RC.

True or False? Using a resistor with a very low resistance is a good safety choice, since it makes the capacitor discharge fastest.

False. A low resistance causes a very high current , which can make the wires dangerously hot and increase the risk of a short circuit or fire. A large resistance should be used so the discharge is slow enough to time accurately and safely.

Why should a capacitor be handled carefully and fully discharged after the experiment?

It can still retain charge after the power is removed, which could cause an electric shock .

Define linearisation , in the context of capacitor discharge graphs.

Transforming an exponential decay equation into the form y = mx + c (e.g. by taking natural logs), so it can be plotted as a straight line and the gradient and intercept used to find RC and Q 0 , V 0 or I 0 .

On a graph of ln(V) against t for a discharging capacitor, the gradient of the straight line is equal to .......... .

On a graph of ln(V) against t for a discharging capacitor, the gradient of the straight line is equal to −1/RC .

On a linearised ln(Q) against t graph for capacitor discharge, what does the y-intercept represent?

ln Q 0 — the natural log of the initial charge.

International A Level (IAL)PhysicsEdexcelFlashcards4. Further Mechanics, Fields & ParticlesCapacitance

Capacitance (Edexcel International A Level (IAL) Physics): Flashcards

Exam code: YPH11

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FrontCapacitance
Define capacitance.
FrontCapacitance
What equation links capacitance, charge and potential difference?
BackCapacitance
$C = \frac{Q}{V}$
C = capacitance (F)
Q = charge stored on the plates (C)
V = potential difference across the plates (V)

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Cards in this collection (34)

Define capacitance.
Capacitance is the charge stored per unit potential difference between the plates of a capacitor.
Define the farad.
The farad (F) is the SI unit of capacitance. Since 1 F is a very large unit, capacitance is usually quoted in microfarads (μF), nanofarads (nF) or picofarads (pF).
What equation links capacitance, charge and potential difference?
$C = \frac{Q}{V}$
- C = capacitance (F)
- Q = charge stored on the plates (C)
- V = potential difference across the plates (V)
For a fixed potential difference, how does increasing a capacitor's capacitance affect the charge it stores?
The charge stored increases. The greater the capacitance, the greater the charge stored for a given potential difference.
A capacitor is commonly made of two conductive metal plates separated by a .........., which prevents charge flowing between them.
A capacitor is commonly made of two conductive metal plates separated by a dielectric, which prevents charge flowing between them.
True or False?
The charge Q in the capacitance equation is the total charge that has flowed around the circuit.
False.
Q is the charge stored on the plates of the capacitor, not the total charge that has flowed through the circuit.
Define the electrical energy stored by a charged capacitor.
The work done by the power supply in moving charge onto the plates of the capacitor, which becomes stored as electrical potential energy.
State the three equivalent equations for the energy stored, W, by a capacitor.
$W = \frac{1}{2} Q V$
$W = \frac{1}{2} C V^{2}$
$W = \frac{Q^{2}}{2 C}$
What does the area under a potential difference-charge (V-Q) graph for a capacitor represent?
The energy stored (work done) by the capacitor. It is equal to the area of the right-angled triangle under the line, $W = \frac{1}{2} Q V$
On a graph of charge against potential difference for a capacitor, the gradient represents ...........
On a graph of charge against potential difference for a capacitor, the gradient represents 1 / capacitance (1/C).
True or False?
Adding the first few electrons onto the negative plate of a capacitor requires more work than adding the last few.
False.
Electrostatic repulsion increases as the plate becomes more negatively charged, so more work is needed to add charge later in the charging process, not the first few electrons.
A 1500 μF capacitor charges from 10 V to 30 V. Calculate the change in energy stored.
$Δ W = \frac{1}{2} C (V_{2}^{2} - V_{1}^{2})$
$Δ W = \frac{1}{2} \times (1500 \times 10^{- 6}) \times (30^{2} - 10^{2})$
$Δ W = 0.6 J$
Define the time constant, τ, of a discharging capacitor.
The time constant is the time taken for the charge, current or potential difference of a discharging capacitor to decrease to 37% of its original value.
What is the equation for the time constant, τ, of a capacitor?
$τ = R C$
- τ = time constant (s)
- R = resistance of the resistor (Ω)
- C = capacitance of the capacitor (F)
For a charging capacitor, what does the time constant represent?
The time taken for the charge or potential difference of a charging capacitor to rise to 63% of its maximum value.
During charging, how does the current in the circuit change with time?
It starts large and decreases exponentially to zero.
The rate at which a capacitor discharges depends on the .......... of the circuit.
The rate at which a capacitor discharges depends on the resistance of the circuit.
True or False?
During charging, the p.d. across a capacitor increases exponentially in the same way the charging current decreases exponentially.
False.
The p.d. and charge rise from zero towards a maximum value; only the current follows an exponential decay curve during charging.
How does increasing the circuit resistance affect the time taken for a capacitor to discharge?
A higher resistance means the current decreases more slowly, so the capacitor takes longer to discharge.
Define the resolution of a measuring instrument.
The smallest change in a quantity that a measuring instrument can detect. In this practical, the voltmeter has a resolution of 0.1 V and the stopwatch a resolution of 0.01 s.
What is the aim of Core Practical 11?
To calculate the capacitance of a capacitor experimentally, by discharging it through a resistor and measuring how the p.d. varies with time.
What are the independent and dependent variables in this experiment?
Independent: time, t.
Dependent: potential difference, V.
How many voltage readings should typically be taken, and at what time interval?
8–10 readings, taken every 10 s until the p.d. reaches 0 V.
What two quantities are plotted against each other to obtain a straight-line graph for finding capacitance?
ln(V) is plotted on the y-axis against time, *t, on the x*-axis.
The capacitance of the capacitor can be calculated from the .......... of the ln(V) against t graph, since gradient = −1/RC.
The capacitance of the capacitor can be calculated from the gradient of the ln(V) against t graph, since gradient = −1/RC.
True or False?
Using a resistor with a very low resistance is a good safety choice, since it makes the capacitor discharge fastest.
False.
A low resistance causes a very high current, which can make the wires dangerously hot and increase the risk of a short circuit or fire. A large resistance should be used so the discharge is slow enough to time accurately and safely.
Why should a capacitor be handled carefully and fully discharged after the experiment?
It can still retain charge after the power is removed, which could cause an electric shock.
Define linearisation, in the context of capacitor discharge graphs.
Transforming an exponential decay equation into the form y = mx + c (e.g. by taking natural logs), so it can be plotted as a straight line and the gradient and intercept used to find RC and Q₀, V₀ or I₀.
State the exponential decay equations for charge, potential difference and current during capacitor discharge.
$Q = Q_{0} e^{- t / R C}$
$V = V_{0} e^{- t / R C}$
$I = I_{0} e^{- t / R C}$
What straight-line equation is obtained by taking the natural log of the charge decay equation?
$\ln Q = - \frac{1}{R C} t + \ln Q_{0}$
On a graph of ln(V) against t for a discharging capacitor, the gradient of the straight line is equal to ...........
On a graph of ln(V) against t for a discharging capacitor, the gradient of the straight line is equal to −1/RC.
On a linearised ln(Q) against t graph for capacitor discharge, what does the y-intercept represent?
ln Q₀ — the natural log of the initial charge.
True or False?
The initial current I₀ for a discharging capacitor can be found using I₀ = V₀ / C.
False.
By Ohm's law, the initial current is $I_{0} = \frac{V_{0}}{R}$ , using the resistance, not the capacitance.
A 10 mF capacitor is fully charged by a 12 V power supply, then discharged through a 1 kΩ resistor. Calculate the discharge current after 15 s.
$I_{0} = \frac{V_{0}}{R} = \frac{12}{1000} = 0.012 A$
$I = I_{0} e^{- t / R C} = 0.012 \times e^{- 15 / (1000 \times 0.01)} = 0.012 \times e^{- 1.5}$
$I = 2.7 \times 10^{- 3} A = 2.7 mA$

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